The present invention relates to the field of microwave thermal therapy of tissue. In particular, the present invention relates to a microwave generating device having a high performance dual directional coupler for precisely measuring forward and reflected power in a microwave antenna transmission line.
The prostate gland is a complex, chestnut-shaped organ which encircles the urethra immediately below the bladder. Nearly one-third of the prostate tissue anterior to the urethra consists of fibromuscular tissue that is anatomically and functionally related to the urethra and bladder. The remaining two-thirds of the prostate is generally posterior to the urethra and is comprised of glandular tissue.
This relatively small organ, which is the most frequently diseased of all internal organs, is the site of a common affliction among older men: BPH (benign prostatic hyperplasia). BPH is a nonmalignant, bilateral nodular expansion of prostate tissue in the transition zone, a periurethral region of the prostate between the fibromuscular tissue and the glandular tissue. The degree of nodular expansion within the transition zone tends to be greatest anterior and lateral to the prostatic urethra, relative to the posterior-most region of the urethra. Left untreated, BPH causes obstruction of the urethra which usually results in increased urinary frequency, urgency, incontinence, nocturia and a slow or interrupted urinary stream. BPH may also result in more severe complications, such as urinary tract infection, acute urinary retention, hydronephrosis and uraemia.
Traditionally, the most frequent treatment for BPH has been surgery (transurethral resection). Surgery, however, is often not an available method of treatment for a variety of reasons. First, due to the advanced age of many patients with BPH, other health problems, such as cardiovascular disease, can warrant against surgical intervention. Second, potential complications associated with transurethral surgery, such as hemorrhage, anesthetic complications, urinary infection, dysuria, incontinence and retrograde ejaculation, can adversely affect a patient's willingness to undergo such a procedure.
A fairly recent alternative treatment method for BPH involves microwave thermal therapy, in which microwave energy is employed to elevate the temperature of tissue surrounding the prostatic urethra above about 45.degree. C., thereby thermally damaging the tumorous tissue. At temperatures above about 45.degree. C., healthy tissue is also thermally damaged. Delivery of microwave energy to tumorous prostatic tissue is generally accomplished by a microwave antenna-containing applicator, which is positioned within a body cavity adjacent the prostate gland. The microwave antenna, when energized, heats adjacent tissue due to molecular excitation. The heat generated by the antenna is concentrated about the antenna in a generally cylindrically symmetrical pattern which encompasses and necroses tumorous tissue as well as healthy intraprostatic tissue to some degree. The necrosed intraprostatic tissue is subsequently reabsorbed by the body, thereby relieving an individual from the symptoms of BPH.
This microwave treatment method is derived from a treatment for prostatic cancer known as hyperthermia, in which microwave energy is supplied by a microwave antenna to the prostate to elevate the temperature of surrounding tissue to between about 43.degree. C. to 45.degree. C. Within this temperature range, healthy, well-vascularized tissue is unharmed because of the circulatory system's ability to effectively dissipate the heat by convection. Cancerous tissue, on the other hand, as reduced vascularity, which restricts its ability to adjust to the heat. Thus, heat concentrated in the region of the cancerous tissue is sufficient to necrose the cancerous tissue, yet insufficient to harm adjacent healthy tissue.
Microwave thermal therapy, because of its higher temperatures (above about 45.degree. C.), provides the advantage of shortening a treatment session's duration as compared to that of hyperthermia with its lower temperatures (between about 43.degree. C. and 45.degree. C.). An undesirable consequence of microwave thermal therapy, however, is the adverse effect the higher temperatures have on healthy tissue adjacent the diseased area of the prostate, particularly the urethra, the ejaculatory duct and the rectum. The dilemma of selectively heating and necrosing only tumorous prostatic tissue by microwave thermal therapy has been successfully addressed in U.S. Pat. No. 5,413,588, entitled DEVICE FOR ASYMMETRICAL THERMAL THERAPY WITH HELICAL DIPOLE MICROWAVE ANTENNA, and U.S. Pat. No. 5,330,518, entitled METHOD FOR TREATING INTERSTITIAL TISSUE ASSOCIATED WITH MICROWAVE THERMAL THERAPY.
Antennas which have been used for hyperthermia have a variety of inadequacies which preclude their application to microwave thermal therapy. First, such antennas often generate heat in two forms: microwave energy and heat energy due to resistive losses of the antenna. The efficiency of these antennas has historically not been of much concern due to the relatively low amount of energy used to generate temperatures of between about 43.degree. C. to 45.degree. C. and the lack of any adverse effect these temperatures had on healthy tissue. Furthermore, it is known in the art that the shape and size of a radiation pattern generated by some microwave antennas are in part a function of how deeply the antenna is inserted into the tissue. Prior microwave dipole antennas used for hyperthermia have been unable to provide a predictable heating pattern within tissue due to the variable effects caused by the depth of insertion of the antennas into the tissue. Finally, the radiation length of these antennas has not been easily variable to accommodate the varying sizes of prostates requiring treatment. The antenna designs of the prior art relating to hyperthermia, therefore, have proven unsatisfactory for microwave thermal therapy and its attendant higher temperatures.
The objective of microwave thermal therapy is to reduce the length of a treatment session and to selectively heat and necrose only undesirous tissue, while sparing, to the greatest extent possible, adjacent healthy tissue. In order to avoid damage to tissues immediately adjacent the microwave antenna-containing applicator (i.e., the urethra, the ejaculatory duct and the rectum), it is essential that the resistive losses of the antenna be reduced or optimally eliminated. The ability to eliminate resistive losses and utilize only microwave energy to heat a targeted tissue area permits a cooling system, such as that described in the above-referenced patents, to maintain safe temperatures adjacent to the applicator by absorbing and conveying away any excess heat conducted to the urethral tissues. In addition, an antenna capable of producing a predictable, yet selectively variable size heating pattern aids in achieving an effective treatment of undesirous tissue while minimizing harm to healthy tissue. An antenna with the above-described characteristics is described in U.S. Pat. No. 5,300,099, entitled GAMMA MATCHED HELICAL DIPOLE MICROWAVE ANTENNA, which is hereby incorporated by reference.
To further optimize the performance of the microwave energy delivery system, it is necessary to provide the capability to precisely detect forward power delivered to the microwave antenna and reflected power from the microwave antenna. There are several control functions that maybe enhanced by an improved dual-directional coupler for more precisely measuring forward and reflected power. For example, the microwave energy delivery system maybe designed to shut down upon detection of reflected power greater than a predetermined threshold, such as 10%, in order to safeguard the system against excessive heat buildup along the antenna and associated transmission line. Automatic frequency adjustment may be provided in the microwave generating source based on reflected power measurements, to optimize the impedance match and efficiency of the antenna within the frequency range available for therapy (e.g., 902-928 megahertz). The impedance of the microwave antenna may be dynamically adjustable according to reflected power measurements, as described in copending U.S. Application Ser. No. 08/621,634 filed Mar. 26,1996 for VOLTAGE CONTROLLED VARIABLE TUNING ANTENNA by E. Rudie. These and other schemes for controlling the microwave energy delivery system illustrate the utility and necessity of precise forward and reflected power detection capability.